The present invention relates to novel polyethylene resin compositions containing an inorganic calcium compound filler and having excellent internal tearing resistance and processibility. More specifically, the present invention relates to a novel polyethylene resin composition obtained by incorporating into a polyethylene having particular molecular weight characteristics one or more inorganic calcium compound selected from a group consisting of calcium sulfite, calcium sulfate and calcium carbonate.
It is well known in the art that polyethylene products having increased mechanical strength, particularly rigidity, may be obtained by incorporating an inorganic filler into polyethylene. In this case, however, the tenacity or tensile strength of the product tends to be reduced when its rigidity is increased. In particular, the tenacity of the product obtainable is extremely reduced if the inorganic filler is added to polyethylene in excess of a certain amount. This reduction of the tenacity is recognized as an extreme reduction of the internal tearing resistance in such products as films and sheets, so that it constitutes a great disadvantage in the use of these materials in industry.
In order to overcome this deficiency and produce polyethylene articles having improved mechanical properties, the inventors of the present invention had earlier proposed a method of preparing particular polyethylene blends by incorporating calcium sulfite as the filler. However, the internal tearing resistance of films and sheets produced from such materials was found to greatly differ between longitudinal and lateral directions. By way of example, films and sheets obtained from the aforementioned polyethylene blends through calendering had internal tearing resistance in the direction of calendering (or longitudinal direction) as low as only a fraction of that in the direction transverse to the calendering direction (or lateral direction). Therefore, the use of these films and sheets is limited due to the necessity of differentiating between their longitudinal and lateral directions in their use. In addition, it is rather difficult to obtain a homogeneous blend of polyethylene and inorganic filler so that processibility of the blend is inferior. Therefore, the product is prone to pores and pin holes. Also, filler particles are likely to remain therein in an agglomerated form so that it is difficult to obtain a high quality product.